As the density and complexity of microcircuits continue to increase, the photolithographic processes used to print circuit patterns becomes more and more challenging. Previous technologies and thinking in the art has required denser and more complex patterns to achieve the formation of the denser circuits consisting of smaller pattern elements packed more closely together. Such patterns push the resolution limits of available lithography tools and processes and place ever increasing burdens on the photolithography processes used to form the many layers of a semiconductor wafer design pattern.
One of the most time consuming and labor intensive tasks undertaken in the lithography cell of a high productivity semiconductor manufacturing plant is that of ensuring good quality alignment performance of a stepper or scanner with minimal impact on fleet productivity. The basic problem resides in the fact that even the most advanced patterning exposure tools possess an intrinsic unique pattern placement error signature, both at the full wafer (sometimes called grid) level and at the individual field (sometimes call shot) level. This is due to residual imperfections in both the optical and mechanical systems of the patterning tool (also referred to herein as an exposure system) which differ from tool to tool, varying by tool identity, by tool model, by tool generation, by tool vendor, and even by tool component and illumination conditions. In order to meet ever shrinking alignment control requirements, the exposure tools require more and more sophisticated control methodologies. In an effort to meet these demands, exposure systems include an ever increasing array of adjustment features with more and more degrees of freedom, all directed toward error compensation.
In an ordinary process an exposure system is obtained by an end user and then calibrated such that it performs within its manufacturer specifications. Only then can such systems take their place in the manufacturing fleet. However, the inventors point out that even though such systems are calibrated to within manufacturer specifications, each tool demonstrates some degree of pattern distortion and misalignments making it imperfect. Ordinarily such imperfections are not particularly troublesome. However, with the pressure to obtain ever shrinking feature sizes and the associated need for greater precision, such systems are under pressure to demonstrate improved precision. Thus, increasingly even systems calibrated to manufacturer specifications are under increased pressure for greater fidelity. The presence of these residual errors can be compounded when combined with other tools which have their own intrinsic errors.
Accordingly, metrology tools are currently used in the art to measure and quantify errors, distortions, and misalignments in each exposure system. Commonly, the exposure systems will be calibrated using highly precise test wafers featuring many alignment targets and a peerless surface. The exposure systems are used to form lithographic patterns on the test wafers. The test wafers are then subject to metrology testing (overlay alignment metrology and the like) to determine the degree of fidelity possible with each exposure system. The degree of error present in each machine is determined.
It turns out that machines demonstrate a few general categories of error propagation. Accordingly, machines having similar error propagation properties are typically grouped together so that pattern alignment can be maintained to a reasonable degree. This principle is depicted in the extremely simplified illustration of FIG. 1. An intended pattern 101 is depicted here as a square pattern. A fleet of exposure systems (A, B, C, D, E, F) is also shown. Each exposure system includes its own distortion signature causing it to deviate from a perfect replication of the intended pattern 101. As mentioned above, the systems frequently demonstrate distortion signatures that are similar to each other. For example, exposure systems A, B, & C of Group 1 have somewhat similar distortions signatures. Also, exposure systems D, E, & F of Group 2 have somewhat similar distortions signatures. However, it is noted that the signatures of the Group 1 systems vary rather more substantially from the signatures of the Group 2 systems. As a result, the Group 1 systems generally are used together and the Group 2 systems are used together. For example, in fabricating lithography layers on a lot of wafers, layer one is formed using system A, layer two is then formed using system B, and layer three is then formed using system C. Thus, relatively good alignment can be maintained using the systems grouped this way. There are some drawbacks to such a system. For example, when a fourth layer is desired to be formed, the wafer is loaded again onto system A and a fourth layer is formed. However, if none of the Group 1 tools are available (i.e., they are currently in use) the process has a bottleneck. The Group 2 tools can not be used because of the variance in signature between the Group 1 and Group 2 systems. Thus, it is possible, for the process to come to a halt, and additionally, the Group 2 systems may lie unused for an extended period of time. Although, on its surface this may seem like a relatively minor problem, one must consider that in many cases, the exposure systems cost $40,000,000 or more each. The cost for idling such an expensive tool is astronomical. Currently, that is the current state of the art.
In some cases adjustments at the set-up stage can be used to harmonize the distortion caused by each machine as much as possible to attempt to overcome the intrinsic mismatches between different exposure tools in the fleet. In some existing methods one may perform so-called “mix & match” activity during the initial ramp up of an exposure fleet prior to, or in parallel, to bringing exposure systems on-line in a manufacturing environment. Since no “absolute” grid reference exists, back to which the relative displacements of the individual exposure tools can be compared, a “golden exposure tool” and a “golden reticle” or both may be selected, and the golden tool and reticle are then used in various scenarios to generate a mix & match database of discrepancies for each exposure tool relative to the golden exposure tool. This technique generally require a lengthy sequence of exposures and subsequent processing of test wafers, followed by intensive high density overlay metrology using a tool such as the Archer AIM overlay metrology tool manufactured by KLA-Tencor on the test wafers. A description of such a procedure is given in the following reference: S. J. DeMoor, J. M. Brown, J. C. Robinson, S. Chang, and C. Tan, “Scanner overlay mix and match matrix generation: capturing all sources of variation” Proc. SPIE Int. Soc. Opt. Eng. 5375, 66 (2004), which document is incorporated herein by reference it its entirety for all purposes. This practice suffers from a number of deficiencies, including time consuming labor intensive non-automated analysis, risk of errors, and that over time, the database can become an inaccurate reflection of the current status of the tool set due to drifts or maintenance induced modifications.
Improved methods for optimizing alignment in a fleet of exposure systems is needed. Among other things, this disclosure seeks to provide solutions to this problem. Accordingly, the embodiments of invention present substantial advances over the existing methodologies and overcome many limitations of the existing pattern fabrication arts. These and other inventive aspects of the invention will be discussed herein below.